The need for reconfigurable antennas and other radio frequency (RF) structures is great. In an era of crowded frequency bands, diverse requirements for multi-frequency communication, etc., antenna structures able to perform in one or more bands or with switchable directionability characteristics are of great interest. One solution to the problem is the use of reconfigurable antennas or other structures (e.g., reflective structures). Generally speaking, these are antennas or associated resonant structures which may have their frequency and/or their directional characteristics altered so as to perform in one or more frequency bands and/or with one or more directionability patterns.
Switched antenna elements have been used for some time, portions of the structures being connected and disconnected by a switch. PIN diodes and GaAs Field Effect Transistors (FETs) have been used to perform these switching operations. The devices typically require a bias current, making their use cumbersome. The advent of microelectromechanical systems (MEMS) has allowed the creation of ultra-small switches. The introduction of MEMS switches has created new possibilities in the RF communications field. For example, multiple ground planes behind a single radiating element may be switched in or out of the circuit using an array of MEMS switches. The MEMS switches can be constructed as bi-stable devices and are switched by the application of an electrical voltage to an input terminal. Of course, any DC voltage source may be used to activate the MEMS switches.
In high frequency (i.e., microwave, millimeter wave, etc.) applications, the introduction of copper or other conductive materials into or near an RF structure may have an undesirable effect. Added wires and conductors may scatter the RF fields around antennas thereby distorting the antenna radiation patterns or affecting their impedance. If the switch control wires can be concealed by the antenna elements or their RF feeds, then the interference with the operation of the antenna can be minimized. However, only a few antenna elements allow embedding of the control lines.
To address this problem, strategies have been developed to use a photovoltaic cell to generate a DC switching voltage for the MEMS switch. A laser light source and optic fiber then are used to activate the MEMS switch. Laser light shining on several photovoltaic cells in series creates a voltage that causes the MEMS switch to change state; a passive antenna element or other structure connected thereto is switched in or out of the circuit.
In some applications, the running of fiber optic strands from a laser light source to the array of MEMS switches is not practical. Also, if the switches must be enclosed in an opaque material, then neither visible nor infrared (IR) light can be used to activate them effectively.